Messages in this thread Patch in this message | | | Date | Mon, 10 Sep 2018 14:27:27 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v6 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active |
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On Fri, Sep 07, 2018 at 12:57:30PM -0500, Brijesh Singh wrote: > Currently, the per-cpu pvclock data is allocated dynamically when > cpu > HVC_BOOT_ARRAY_SIZE.
Well no, you need to write this correctly - what is "cpu > HVC_BOOT_ARRAY_SIZE" ?!
( I know what it is but I know it only because I've looked at that code before. )
So no, please explain it in English not in code.
> The physical address of this variable is > shared between the guest and the hypervisor hence it must be mapped as > unencrypted (ie. C=0) when SEV is active.
This sentence is a good example about how to explain stuff in commit messages.
> The C-bit works on a page,
"The C-bit determines the encryption status of a 4K page."
> hence we will be required to perform a
Use passive tone in your commit message: no "we", etc...
> full 4k page allocation to store a single 32-byte pvclock variable. It > will waste fairly sizeable amount of memory since each CPU will be doing
"... will waste *a* fairly sizeable amount of ..."
> a separate 4k allocation.
Start new paragraph here and use passive tone.
> Let's define a second array for the SEV case to > statically allocate for NR_CPUS and put this array in .data..decrypted
NR_CPUS needs explaining for the unenlightened reader. Also,
"... put this array in *the* .data..decrypted section... "
> section so that its mapped with C=0 during boot.
<---- newline here.
> The .data..decrypted > section has a big chunk of memory that is currently unused. And since > second array will be used only when memory encryption is active hence
"... since *the* second array... "
s/hence //
> free it when encryption is not active. > > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com> > Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com> > Cc: Tom Lendacky <thomas.lendacky@amd.com> > Cc: kvm@vger.kernel.org > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Borislav Petkov <bp@suse.de> > Cc: "H. Peter Anvin" <hpa@zytor.com> > Cc: linux-kernel@vger.kernel.org > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Sean Christopherson <sean.j.christopherson@intel.com> > Cc: kvm@vger.kernel.org > Cc: "Radim Krčmář" <rkrcmar@redhat.com> > --- > arch/x86/include/asm/mem_encrypt.h | 4 ++++ > arch/x86/kernel/kvmclock.c | 14 ++++++++++++++ > arch/x86/kernel/vmlinux.lds.S | 3 +++ > arch/x86/mm/init.c | 3 +++ > arch/x86/mm/mem_encrypt.c | 10 ++++++++++ > 5 files changed, 34 insertions(+) > > diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h > index 802b2eb..cc46584 100644 > --- a/arch/x86/include/asm/mem_encrypt.h > +++ b/arch/x86/include/asm/mem_encrypt.h > @@ -48,11 +48,13 @@ int __init early_set_memory_encrypted(unsigned long vaddr, unsigned long size); > > /* Architecture __weak replacement functions */ > void __init mem_encrypt_init(void); > +void __init free_decrypted_mem(void);
Proper prefixing:
"mem_encrypt_free_decrypted"
or so
> bool sme_active(void); > bool sev_active(void); > > #define __decrypted __attribute__((__section__(".data..decrypted"))) > +#define __decrypted_aux __attribute__((__section__(".data..decrypted.aux"))) > > #else /* !CONFIG_AMD_MEM_ENCRYPT */ > > @@ -80,6 +82,7 @@ static inline int __init > early_set_memory_encrypted(unsigned long vaddr, unsigned long size) { return 0; } > > #define __decrypted > +#define __decrypted_aux > > #endif /* CONFIG_AMD_MEM_ENCRYPT */ > > @@ -93,6 +96,7 @@ early_set_memory_encrypted(unsigned long vaddr, unsigned long size) { return 0; > #define __sme_pa_nodebug(x) (__pa_nodebug(x) | sme_me_mask) > > extern char __start_data_decrypted[], __end_data_decrypted[]; > +extern char __start_data_decrypted_aux[]; > > #endif /* __ASSEMBLY__ */ > > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c > index 376fd3a..6086b56 100644 > --- a/arch/x86/kernel/kvmclock.c > +++ b/arch/x86/kernel/kvmclock.c > @@ -65,6 +65,15 @@ static struct pvclock_vsyscall_time_info > static struct pvclock_wall_clock wall_clock __decrypted; > static DEFINE_PER_CPU(struct pvclock_vsyscall_time_info *, hv_clock_per_cpu); > > +#ifdef CONFIG_AMD_MEM_ENCRYPT > +/* > + * The auxiliary array will be used when SEV is active. In non-SEV case, > + * it will be freed by free_decrypted_mem(). > + */ > +static struct pvclock_vsyscall_time_info > + hv_clock_aux[NR_CPUS] __decrypted_aux;
Hmm, so worst case that's 64 4K pages:
(8192*32)/4096 = 64 4K pages.
Now, the real question from all this SNAFU is, why can't all those point to a single struct pvclock_vsyscall_time_info and all CPUs read a single thing? Why do they have to be per-CPU and thus waste so much memory?
-- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --
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